Investigations under this project have resulted in a methodology and associated technology for assessment of the instantaneous elasticity in living active muscle. The results from application of the method in recent experiments on frog muscle suggest that stiffness changes which occur in muscle immediately after stimulation and before external tension production are attributable to formation of cross-bridges between the myofilament arrays. The time course of stiffness change has been determined and related to corresponding changes in muscle tension output. These results also have been examined relative to existing mechanical models of muscle and the method is expected to be applicable to the determination of rate constants for certain excitation-contraction process.